An equalizer system in general compensates frequency dependent losses that a signal experiences when passing through a transmission channel. Transmission channels include, but are not limited to, a wire, a pair of wires, an optical fibre, the reading and writing channels of a storage device like a hard-disc or optical disc, a wireless connection such as a point-to-point or diffuse infra-red or radio connection. A pair of wires includes a twisted pair, a twinax coax or a differential transmission line on a printed circuit board.
The compensation level of an equalizer system in general can be self-adaptive, fixed or programmable e.g. by a voltage or via a set of switches. A self-adaptive equalizer system continuously estimates the matching compensation level. It typically includes an adaptable filter, a control loop and an output reconstruction unit.
EP-1392001 describes how to organise a control loop in an equalizer system such that self-adaptation is achieved, independently from the transmit amplitude and the transmitted bit pattern. A feed-back control signal is generated from the equalised output of an equalizer filter. Depending on whether the output signal has been under- or over-compensated, the feed-back control signal increases or decreases, such that after a reasonable time the feed-back control signal converges to a value where matched compensation is reached. The control loop is formed by a first means for measuring a short-term-amplitude signal of the output signal, a second means for measuring a long-term-amplitude signal of the output signal and a comparator means for comparing the short-term-amplitude signal and the long-term-amplitude signal, and for determining the evolution of the feed-back control signal.
U.S. Pat. No. 5,841,810 describes a way to arrange multiple adaptive filter stages in an adaptive filter. The plurality of filter stages have a common equalisation control signal that has a magnitude that corresponds to the communications path transfer function, with each adaptive filter stage transfer function being an approximate inverse of a transfer function that corresponds to a portion of the input data signal communications path. The compensation thus is based on the ideal transfer function of the communications path.
U.S.-2002/0034221 discloses a communications receiver that has multiple stages each having a transfer function 1+Ki[fi(jω)], wherein the Ki vary with a sequential gain control methodology. This document thus teaches to compensate by making a sum per stage of the unity input signal linearly added to a function that has higher frequency gain. This known method makes multiple tuning signals in circuitry using many comparators and is relative complex. It is not suited for low voltage operation nor for implementation on a small chip area using small transistors that have large input offset mismatches.
WO 2004/073274 describes how to organise an adaptive equalizer filter with multiple stages that can operate at low-voltage, and whereby the stage that is being tuned can operate in a non-linear way, still giving sufficient restoration of a transmitted digital data signal. Multiple tuning circuits generate tuning signals. Each tuning signal can typically induce higher frequency gain up to a limited level, e.g. +5 dB, at the upper data frequency for compensation of high frequency losses in the connected transmission channel. Several tuning signals can tune one adaptive amplifying compensation stage. In its adaptive amplifying compensation stage the tuning signal can generate through its tuning function, non-linear small-signal and large-signal transfer behaviour. However, by limiting the amount of higher frequency gain to maximum +8 dB per tuning function, and by having only one tuning function active at a time the resulting deterministic jitter remains tolerable.
A difficulty with the above-mentioned state-of-the-art adaptive and self-adaptive equalizer filters and systems is that they estimate the losses in the channel and then compensate these losses by matched complementary amplification. The precision with which this loss-level is being estimated and with which the compensation is being set, largely determines the quality of the restored bit-stream at the output of the adaptive equalizer filter in terms of achieved jitter performance.
The above-described prior art adaptive and self-adaptive equalizer filters only teach how a multi-stage equalizer system can be conceived that compensates signal modifications introduced by a transmission channel for a limited number of different types and lengths of transmission channels.